This invention relates to endoscopes or borescopes and is more particularly directed to an illumination system comprising a fiber optic light guide that carries light to the distal end of the endoscope or borescope insertion tube and a distributing lens system capable of providing wide angle illumination of a target.
In a typical endoscope or borescope, a lens system, comprised of one or more conventional spherical lenses, is disposed in the head of the endoscope or borescope for spreading the light carried by the fiber optic light guide. A spherical lens has a generally bell-shaped light distribution. Hence, when the illumination angle becomes wider to correspond with a wide viewing angle, the difference in illumination between the center and the edges becomes greater. One reason for this edge illumination dropoff is the fourth-power-of-cosine law that applies to spherical lenses.
In addition to this infirmity, positive spherical lenses tend to focus light, and can cause a dot or mesh pattern of illumination to appear on the target, from the light emanating from the optical fibers, when an even blanket of light would be desired instead.
A number of previous attempts have been made to resolve the problem of wide angle distribution of light. In U.S. Pat. No. 4,415,240, a cylindrical reflecting member is placed between the distal end of the fiber optic light guide and the lens distribution system. However, the lenses of that system are positive spherical lenses. U.S. Pat. No. 4,580,522 describes a spherical lens illumination system in which a bar-shaped member is embedded at the center of the exit end of the fiber optic light guide, which reduces the light intensity at the center of the target. However, no attempt is made in either case to adjust the light distribution properties of the lenses themselves, so these previously proposed systems have not been able to provide the desired flat illumination distribution for extremely wide illumination angles.